1. Field of the Invention
The invention relates to highly wear and corrosion resistant, powder metallurgy tool steel articles and to a method for their production by compaction of nitrogen atomized, prealloyed high vanadium powder particles. The articles are characterized by exceptionally high metal to metal wear resistance, which in combination with their good abrasive wear resistance and corrosion resistance, makes them particularly useful in machinery used for processing reinforced plastics and other abrasive or corrosive materials.
2. Background of the Invention
Basically, there are three types of wear that can occur, often in combination, in the barrels, screws, valves, molds, and other components used in processing reinforced plastics and other aggressive materials. They include metal to metal wear caused in areas where the metal components come into direct contact during operation, abrasive wear caused by continued contact at high pressures of the components with hard particles in the process media, and corrosive wear caused by acids or other corrodents either originally present or released from the process media at elevated temperatures of operation. To perform satisfactorily, the articles used in processing these materials must be highly resistant to these forms of wear. In addition, they must possess sufficient mechanical strength and toughness to withstand the stresses imposed during operation. Further, they must be readily machined, heat treated, and ground to facilitate the manufacture of parts with the required shape and dimensions.
A wide range of materials have been evaluated for the construction of the components employed in the processing of reinforced plastics and other abrasive or corrosive materials. They include chromium plated alloy steels, conventional high chromium martensitic stainless steels such as AISI Types 440B and 440C stainless steels, and a number of high chromium martensitic stainless steels produced by powder metallurgical methods. The compositions of this latter group of materials are broadly similar to those of the conventional high chromium martensitic stainless steels, except that greater than customary amounts of vanadium and carbon are added to improve their wear resistance. The high chromium, high vanadium, powder metallurgy stainless steels, such as CPM 440V disclosed on page 781 in Volume 1 of the 10th Edition of the ASM Metals Handbook and MPL-1 disclosed in recent publications, clearly outperform conventional steels in plastic processing, but none of these materials fully meet all the needs of the newer plastic processing machinery which cannot accommodate large wear related changes in the geometry of the operating parts and where contamination of the process media by wear debris must be minimized. Of all the required properties, the metal to metal wear resistance of the high chromium martensitic stainless steels made either by conventional or powder metallurgy methods is remarkably low.